1. Field of the Invention
This invention relates to a power semiconductor device having a high breakdown voltage.
2. Description of the Related Art
FIGS. 1A, 1B and 1C respectively show the cross sectional structure of the main portion of a conventional diode having a high breakdown voltage, the distribution of impurity concentration and the distribution of carrier concentration in the ON state. An anode electrode 3 is formed over one surface of a high-resistance base layer 1 formed of n.sup.- -type silicon with a p.sup.+ -type anode layer 2 disposed therebetween and a cathode electrode 5 is formed over the other surface of the base layer 1 with an n.sup.+ -type cathode layer 4 disposed therebetween. In the case of a high breakdown voltage diode having a blocking voltage of approx. 4500 V, the impurity concentration and dimension of each portion are determined such that the impurity concentration of the high-resistance base layer 1 is 1.0.times.10.sup.13 to 1.8.times.10.sup.13 /cm.sup.3, the thickness thereof is 450 to 900 .mu.m, the surface impurity concentration of the p.sup.+ -type anode layer 2 and n.sup.+ -type cathode layer 4 is 1.times.10.sup.19 /cm.sup.3, and the thickness thereof is 14 to 70 .mu.m. In this type of high breakdown voltage diode, an ON voltage of approx. 2.6 V can be obtained with a current of 100 A/cm.sup.2. The high breakdown voltage characteristic can be attained by forming the junction end (termination) portion in the bevel structure.
In the conventional high breakdown voltage diode, a large amount of carriers are accumulated in the high-resistance base layer 1 in the highly injected state. The carrier distribution as shown in FIG. 1C is obtained. Particularly, high impurity concentration is exhibited in portions near the n.sup.+ -type cathode layer 4 in which electrons are injected and the p.sup.+ -type anode layer 2 in which holes are injected. When a large amount of carriers are thus accumulated, a large reverse recovery current flows in the OFF time in which a reverse bias voltage is applied. For example, in the case of the above element parameters, a reverse recovery current of approx. 10 mA (100 A/cm.sup.2) flows when it is turned off with an application voltage of 1000 V and current variation rate di/dt=200 A/.mu.s. Thus, large power is consumed by the reverse recovery current to generate heat. This prevents attainment of a high-speed switching operation.
As a method of improving the reverse recovery characteristic of the high breakdown voltage diode, it is known that a method of lowering the surface impurity concentration of the p.sup.+ -type anode layer and reducing the thickness thereof is effective (for example, IEEE TRANSACTIONS OF ELECTRON DEVICES, VOL-23, NO. 8 1976 MASAYOSHI et al, "High Current Characteristics of Asymmetrical P-i-N Diodes Having Low Forward Voltage Drops").
However, lowering the surface impurity concentration of the p.sup.+ -type anode layer makes it difficult to sufficiently lower the resistance of the ohmic contact between the anode layer and the anode electrode. In order to attain the good ohmic contact necessary for a power element, it is necessary to set the surface impurity concentration to approx. 1.times.10.sup.19 /cm.sup.3. Further, when the impurity concentration of the anode layer is lowered and the thickness thereof is reduced, a state in which a depletion layer formed to extend in the anode layer at the time of application of reverse bias voltage reaches the anode electrode is set up and a sufficiently high breakdown voltage characteristic cannot be obtained.
As described above, in the conventional high breakdown voltage diode, a large reverse recovery current is caused to flow in the OFF time by accumulation of carriers in the thick high-resistance base layer, thereby preventing attainment of high-speed switching operation. Further, if the anode layer is formed with relatively low impurity concentration and small thickness in order to reduce the amount of carriers accumulated on the anode side, problems may occur that good ohmic contact cannot be formed and high breakdown voltage characteristic cannot be attained.
Similar problems are not limited to the high breakdown voltage diodes and may occur in other elements such as thyristors and bipolar transistors having the same diode structure.